Make the Post Type filter optional

We've disabled the following to enable usage for Custom Post Types:

```
public function display_post_types_selection() {
// @TODO add more suitable filters
$post_types = get_post_types( array( 'public' => true ) );
```
Any chance you can make this an option please? Perhaps with a disclaimer that not all CPTs may work if they use custom tables if you're concerned about that.

Start by reading display_post_types_selection() and the current get_post_types( array( 'public' => true ) ) call shown in the issue. Done means the post-type filter can optionally include Custom Post Types, with a disclaimer that CPTs using custom tables may not work.

Written by the indexing model from the issue text.